Some of the of biggest special interest groups in Washington are expected to take full advantage of a Supreme Court decision Thursday enabling them to spend millions on attack ads in the 2010 midterm elections, even as the Obama administration and congressional Democrats scramble to close the gaping holes the ruling carved into campaign finance rules.

Thursday’s highly anticipated 5-4 decision in a case brought by the conservative nonprofit group Citizens United reversed decades of law restricting corporations and unions from spending their general funds on ads supporting or opposing candidates. And it left liberals and advocates for stricter campaign finance rules predicting an explosion of corporate-funded ads attacking Democrats.

“We are moving to an age where we won’t have the senator from Arkansas or the congressman from North Carolina, but the senator from Wal-Mart and the congressman from Bank of America,” said Melanie Sloan, executive director of the left-leaning watchdog group Citizens for Responsibility and Ethics in Washington.

Campaign strategists and lawyers who advise corporations, unions and independent political groups on political spending also predicted a surge in ads as a result of the decision.

Ads like those aired by Swift Boat Veterans for Truth attacking 2004 Democratic presidential candidate John Kerry can now be paid for more directly by for-profit or nonprofit corporations or trade groups.

In place of 527s, Ginsberg predicted an expanded role for groups set up under sections 501(c)4 and 501(c)6 of the Internal Revenue Service code, which he said require “meager disclosure requirements of their donors.”

Ginsberg also predicted the decision would be good for consultants who advise outside groups on their spending and media strategies. One Democratic consultant professed to making “tons of sales calls” after the decision, calling it an “economic recovery package” for consultants.

What many strategists and lawyers said they don’t expect to see is American International Group spending millions on ads attacking congressmen who criticized its bonuses or medical firms seeking vengeance on President Barack Obama for pushing to overhaul the nation’s health care system.

Instead, they think deep-pocketed companies seeking to target Obama or congressional Democrats will funnel their cash to existing or yet-to-be-created coalitions — such as the U.S. Chamber of Commerce and the National Rifle Association, Pharmaceutical Research and Manufacturers of America or the National Association of Manufacturers — that are expected to take advantage of the new spending flexibility provided by the ruling.

In recent years, shareholders have independently challenged executives to disclose, explain and justify their participation in partisan politics. For now, that mostly includes making political donations. The scrutiny would intensify significantly if it also meant paid television advertising.

Most corporate leaders of publicly held firms long ago grew leery of playing in politics in a big way lest they put off customers, though the Chamber and other umbrella groups could shield some businesses from exposure by pooling money and taking full credit or blame for how it is spent.

At the same time, campaign finance experts expect there will be some privately held firms with ideological bents that now will engage more directly in campaigns.

“The greatest opportunity for money to flow is corporations giving to trade associations and advocacy groups to have them air the ads,” said Michael Toner, a former Federal Election Commission chairman who advises Republican committees and candidates on campaign finance laws. “They’re the ones that could really benefit from this. The key will be how much money can they amass, and how much are they willing to spend?”

But the new landscape could also benefit unions and big-spending outside groups that tend to support Democrats, such as the Sierra Club and NARAL Pro-Choice America, though such groups tend to have less access to cash than do corporations.

The White House on Thursday joined with Democratic lawmakers, who were facing a treacherous 2010 electoral environment even before the decision, in pledging to push through legislation to minimize the impact of the decision before the midterm elections.

But it’s going to be tough for them to significantly alter the landscape before Election Day — some states hold primaries as early as next month. And there hasn’t been any political will for major campaign finance reform since the 2002 overhaul known as the McCain-Feingold act.

The long-awaited court ruling stems from a lawsuit against the Federal Election Commission brought by Citizens United. It alleged its free speech rights were violated when the FEC moved to block it from using corporate cash to promote and air “Hillary: The Movie,” a feature-length movie harshly critical of then-senator — and current secretary of state — Hillary Clinton during her 2008 campaign for the Democratic presidential nomination.

The FEC asserted that the movie expressly opposed Clinton’s election and therefore was subject to campaign laws that bar the use of corporate cash to air election ads and that require donor disclosure. Citizens United disagreed and sued.

The court divided along ideological lines in the case, with Justice Anthony Kennedy casting the deciding vote and writing the majority opinion. He was joined by Roberts and fellow conservative justices Samuel Alito and Antonin Scalia.

After the ruling, some groups that advocate stricter campaign finance rules called for a constitutional amendment specifying that for-profit corporations are not entitled to First Amendment protections, except for freedom of the press.

Among the more incremental legislative options congressional Democrats and the White House are considering to stem the flow of money into campaigns are proposals to publically finance congressional elections or require corporations to disclose their political activity to shareholders.

And Sen. Chuck Schumer (D-N.Y.) on Thursday pushed the idea of requiring shareholders to vote before a corporation could give money directly to a candidate.

House Speaker Nancy Pelosi (D-Calif.), who blasted the ruling as paving the way for “special-interest dollars to dictate the details of public policy,” said House Democrats would “work with the Obama administration and explore legislative options available to mitigate the impact of this disappointing decision.”

And Obama, in a statement accusing the court of giving “a green light to a new stampede of special interest money in our politics,” said he had instructed his administration “to get to work immediately with Congress on this issue.”

White House ethics lawyer Norm Eisen, the Obama administration’s lead on campaign finance issues, has been in contact with Schumer and Rep. Chris Van Hollen (D-Md.) about tougher campaign finance rules.

The president’s strongly worded statement echoed both Obama’s campaign rhetoric about the need to reduce the role of special interest money in politics and his recent efforts to tap populist anger.

He called the ruling “a major victory for Big Oil, Wall Street banks, health insurance companies and the other powerful interests that marshal their power every day in Washington to drown out the voices of everyday Americans.”

But Obama has largely disappointed advocates for stricter campaign finance rules, who point out he has yet to fulfill a campaign promise to overhaul the presidential public financing system.

“This is a test of the commitment of the White House and congressional leadership toward having reasonable limits on money and politics,” said Craig Holman, a lobbyists for Public Citizen, a nonprofit group that pushes for stricter campaign finance rules.